Endodontic procedures require cleaning of the root canals and canal shaping. Proper cleaning and shaping of the root canals is a crucial factor determining the success of the treatment. In shaping a canal, it is important that the canal wall be smooth and free of “ledges”. Cleaning and shaping of the root canal can be done manually using an endodontic file. Usually several endodontic files of different shapes and/or sizes are required to complete the procedure. Motor driven endodontic files (rotary files) are also known. Rotary files are typically tapered and have one or several helical fluted blades. The usage of rotary files was initially limited to straight canals because the stainless steel files were not flexible enough to withstand rotation in curved canals. This situation changed with the development of endodontic files made from Nitinol which have a flexibility about 6 times higher than that of stainless steel files. Nitinol files can withstand motor driven rotary operation even in curved canals.
Nonetheless the Nitinol rotary files are subjected to significant mechanical stress, and after prolonged use, material fatigue can cause sudden breakage of the file without any prior visible deterioration of the file surface. Nitinol files have also been known to become jammed inside the canal and broken by excess torque applied by the motor. This has lead to the development of dental motors intended for endodontic treatments (“endomotors”) having a low rotational speed (usually 300 rpm to 1,000 rpm) and low torque (typically up to 6 Ncm) with torque control, enabling stopping or reversing the file rotation when a preset torque limit is reached. Another problem associated with motor driven files is self-threading of the rotating file into the canal due to the helical blades on the file surface. The self-threading effect reduces the dentist's ability to control axial advance of the file and may lead to over-instrumentation.
Dental motors with special reciprocating handpieces which generate a rotary oscillation (reciprocation) of the file with a predetermined arc have also been used. The reciprocating handpieces tend to reduce file breakage caused by jammed files since deformation of the file remains within the resiliency limits of the Nitinol. The oscillating motion prevents self threading and improves control of the axial movement of the file. The arc of oscillation (usually 60° to 90°) tends, however, to cause unequal shaping of the canal walls and to create vertical ledges in the canal wall, which impairs the outcome of the treatment. FIG. 2 shows a cross-section of a tooth root 30 in which ledges 31 have formed due to unequal shaping of the canal wall 32.
U.S. Pat. No. 5,944,523 discloses a motor assembly for root canal treatment and other dental applications based on a stepping motor that generates various movements of the file such as a continuous clockwise or counterclockwise rotation, alternating movement and vibration with low angular amplitude.
To achieve alternating movement of the endodontic file with an appropriate rotational speed (several hundred RPM) fast acceleration and deceleration are required. A stepping motor, on the other hand, is only capable of generating a slow acceleration and deceleration. Furthermore, an alternating movement cannot provide full coverage of the root canal wall and will generate ledges.
U.S. Pat. No. 6,293,795 discloses a device in which clockwise rotation of the endodontic file of a relatively large arc alternates with counterclockwise rotation of a smaller arc. In this way, the cutting edges of the file pass over the entire surface of the root canal providing complete coverage of the canal surface and the creation of vertical ledges is prevented. The alternating movement of the file helps to prevent file breakage and to reduce self threading, which is not completely eliminated since the forward arc of file rotation is always larger than the reverse arc.
Root canals have different cross-section shapes which may be divided in two categories: “round” canals and “flat” canals. FIG. 3 shows the cross section of three roots 35, 36 37 having root canals 40, 41 and 42, respectively, that are essentially round in shape. FIG. 3 also shows the cross section of three roots 34, 38 39 having root canals 43, 44 and 45, respectively, that are essentially flat in shape. The broken circles in FIG. 3 show the cross section of the area cut by an endodontic file in relation to the various root canals. It is important that during root canal cleaning and shaping the maximum surface of the root canal walls be covered. Shaping of a “round” canal is relatively simple since endodontic files with different diameters are manufactured and a file or succession of files optimally suiting a specific “round” canal may be selected for treatment. In case of “flat” canals, the situation is more complicated. The round cross-section of the area cut by an endodontic file does not conform well to the elongated cross-sectional shape of the canal leaving large areas of the canal walls untreated. It is possible to “drive” the file along the flat surface of the canal wall so that the cutting edge of the file “shaves” the wall, but control of such movement is very difficult due to self-threading of the file. Moreover, such poorly controlled movement may easily cause jamming and breakage of the file.